This invention relates to a position detection device capable of producing absolute position data at a short period by interpolation and, more particularly, to a position detection device capable of producing incremental pulses at a short period on the basis of such interpolation.
In a position detection device of a phase shift type as disclosed by U.S. Pat. No. 4,754,220, a secondary output signal sin (.omega.t+.theta.) is obtained by shifting the electrical phase angle of a primary ac signal sin .omega.t by an angle .theta. corresponding to a position of an object of detection as shown in FIG. 6 for example, and data 1, 2, 3, . . . of the phase shift amount in this secondary output signal are sampled and held at each period to provide absolute position data of the object of detection. For example, the phase angle is counted from a predetermined phase angle (e.g., 0) of the primary ac signal sin .omega.t to a predetermined phase angle (e.g., 0) of the secondary output signal sin(.omega.t+.theta.) and each count is sampled and held at each sampling timing synchronized with one period of the phase angle (by a sampling pulse SP). In this case, when the object of detection is at a standstill at a certain position, data .theta.1, .theta.2, .theta.3, . . . do not change, indicating the position at which the object is at a standstill. When the object of detection is moving, data .theta.1, .theta.2, .theta.3, . . . obtained at each sampling timing in correspondence to current position of the object of detection at each sampling timing undergo change. The minimum time unit of this change is each sampling timing.
Such position detection device of a phase shift type is generally an absolute position detection device which does not produce an incremental pulse.
There is an example, as disclosed by Japanese Preliminary Utility Model Publication No. 57-168061, in which an incremental pulse is generated by an absolute position detection device. In this device, an incremental pulse is generated in synchronism with a sampling pulse SP of each period of the secondary output signal. The capability of generation of an incremental pulse by an absolute position detection device provides the advantage that applications of the position detection device increase, for the device can be utilized for a control device using an incremental pulse while enjoying advantages proper to the absolute position detection device.
On the other hand, as a known device for generating an incremental pulse, there is a known optical type incremental encoder which reads s pulse pattern of a rotary code plate in an optical manner.
In the above described position detection device of a phase shift type, the minimum time unit of change of absolute position detection data is limited to one period of the sampling pulse SP. As a result, if the speed of moving of the object of detection is high, the value of output absolute position detection data is dispersed with resulting reduction in accuracy. In other words, movement of the object of detection cannot be detected during one period of the sampling pulse SP, however large the distance of movement of the object of detection may be, and this adversely affects accuracy of detection. If, particularly, output of this position detection device is supplied to a control device for utilization therein, resolution of change time of position data supplied from the position detection device to the control device should be as fine as possible from the standpoint of preventing occurrence of beat, since an operation clock of the control device is not synchronized with the sampling timing of the position detection device at all. It is therefore undesirable that the resolution of change time of position data should be limited to one period of the sampling pulse SP as described above.
In generating an incremental pulse by using an absolute position detection device, the system as disclosed by the above mentioned Preliminary Utility Model Publication No. 57-168061 in which an incremental pulse is generated in synchronism with the sampling pulse SP is undesirable because resolution of the incremental pulse is limited to one period of the sampling pulse SP. For this reason, in said Preliminary Utility Model Publication No. 57-168061, it is made one of conditions of detection that the speed of the object of detection should be below a predetermined speed. This however reduces speed response characteristic of the position detection device. If this position detection device is applied to a speed which exceeds the limited speed response characteristic of the device, some pulses will be dropped from the incremental pulse train produced with a result that the number of pulses does not correspond exactly to the amount of displacement.
The known optical type incremental encoder is limited in its speed response characteristic due to its limited pulse production ability. Such incremental encoder generally has a speed response of only about 50 kHz and the speed response cannot be increased to more than about 200 kHz even if the speed response is increased to the highest possible order.